The objective of the proposed research is to understand the molecular biology of regulation of muscle contraction by troponin and calcium. Since calcium regulates many cellular processes, and troponin-like proteins have been isolated from brain, synaptic vesicles, and platelets, an understanding of the mechanism of regulation of muscle contraction will be relevant to all actomyosin-type motile systems and perhaps to other processes controlled by calcium. (1) Experiments are outlined to study the surface of native troponin from rabbit skeletal muscle, isolated and in combination with actin and tropomyosin, using a competitive labelling procedure. The relative reactivity of the epsilon-amino groups of lysine with acetic anhydride will be related to the amino acid sequence of each troponin component. Determination of which residues are "inaccessible" and which are freely "accessible" should elucidate the areas of each protein involved in conformational changes and interaction with other proteins in the thin filament. (2) The myosin DTNB light chain has sequence homologies with troponin-C and binds to troponin-I and T. This complex will be isolated, characterized using electrophoresis and crosslinking reagents, and tested for its ability to confer calcium sensitivity on the actomyosin ATPase. (3) Conformational changes in the actin filament in the presence and absence of tropomyosin, troponin, myosin, and ATP will be studied using the susceptibility of actin to depolymerization by DNase I as a probe.